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period, which allows the authors to estimate the agricultural potential at each place
of the valley. Each household has a demographic functioning, nutritional needs and
a capacity for agricultural work which are implemented in the model. The central
rule is the one that manages the migration of a household. This occurs if the stored
crop added to the year crop is inferior to the nutritional needs of the household. The
household then chooses a new plot; one that can fulfill its needs. If such a plot does
not exist, the household leaves the valley. The first simulations do not manage to
reproduce the population evolution curve. This evolution will only be correctly
reproduced (Figure 4.8) with the condition to introduce heterogeneity in the
demographic and nutritious behaviors of households.
13
However, no simulations
result in the abandonment of the valley: the environmental conditions are actually
such that a small population could be maintained. Therefore, the explanation is to
search in factors that are not taken into account in the model, of social and/or
anthropological order: below a certain population size, the community has taken the
decision to leave the valley [AXT 02, GUM 03].
While recognizing the validity of this model, some authors point out that the
same results could have been achieved by developing a model more parsimonious at
the aggregate level [JAN 09]. However, the interest of this example is to illustrate
the driving role that the interindividual diversity can play, an epistemological
position often defended by the microsimulation modelers.
Figure 4.8.
Population curves of Anazasi households [AXT 02]
13 An example, to clarify: in the first version of the model, a child in a household separates
from his family and forms a new household at the age of 16. In the second version, this action
corresponds to a random selection from a normal distribution of mean 16, i.e. there will be
heterogeneity in the ages of moving out. In the same way, the size of households and their
nutritional needs are supposed to vary.
